1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for allocating subchannels to a plurality of mobile stations in a mobile communication system using Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
Currently, a mobile communication system is evolving from a 3rd generation (3G) mobile communication system into a 4th generation (4G) mobile communication system. The 4G mobile communication system targets efficient interworking and integration of services between a wired communication network and a wireless communication network beyond the simple radio communication services supported by the earlier-generation mobile communication system. In the 4G mobile communication system, standardization is being conducted on the technologies for providing a higher rate data transmission service than that provided in the 3G mobile communication system.
In the mobile communication systems, when a signal is transmitted over a radio channel, the transmission signal suffers multipath interference due to various obstacles located between a transmitter and a receiver. The radio channel having multiple paths is characterized by maximum delay spread of the channel and a transmission period of a signal. When the transmission period of a signal is longer than the maximum delay spread, no interference occurs between consecutive signals, and a frequency characteristic of the channel is given as nonselective fading.
However, when a single-carrier scheme is used for high-speed data transmission having a short symbol period, distortion occurs due to intersymbol interference (ISI), thereby causing an increase in the complexity of an equalizer of a receiver.
Therefore, a system using Orthogonal Frequency Division Multiplexing (OFDM) scheme has been proposed as an alternative for solving an equalization problem in the single-carrier transmission scheme. OFDM is a transmission scheme using multiple carriers, and is a kind of Multi-Carrier Modulation (MCM) in which a serial input symbol sequence is converted into parallel symbol sequences and then modulated with mutually orthogonal subcarriers, or subchannels, before being transmitted.
The first MCM systems appeared in the late 1950s for military high frequency radio communication, and OFDM with overlapping orthogonal subcarriers was initially developed in the 1970s. In view of orthogonal modulation between multiple carriers, OFDM has limitations in actual implementation for systems. In 1971, Weinstein et al. proposed an OFDM scheme that applies DFT (Discrete Fourier Transform) to parallel data transmission as an efficient modulation/demodulation process, which was a driving force for the development of OFDM. Further, the introduction of a guard interval and a cyclic prefix as the guard interval mitigates adverse effects of multipath propagation and delay spread on systems.
Accordingly, OFDM has widely been used for digital data communications such as Digital Audio Broadcasting (DAB), Digital TV Broadcasting, Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (W-ATM). Although hardware complexity was an obstacle to widespread use of OFDM, recent advances in digital signal processing technologies including FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) enable the OFDM implementation.
OFDM, similar to the existing FDM (Frequency Division Multiplexing), boasts of optimum transmission efficiency in high-speed data transmission because it transmits data on subcarriers, while maintaining orthogonality among the subcarriers. The optimum transmission efficiency is further attributed to good frequency use efficiency and robustness against multipath fading in OFDM.
More specifically, overlapping frequency spectrums lead to efficient frequency use and robustness against frequency selective fading and multipath fading. OFDM reduces the effects of the ISI by using guard intervals and enables design of a simple equalizer hardware structure. Further, because OFDM is robust against impulse noise, it is increasingly popular in communication systems.
FIG. 1 is a block diagram illustrating a transmitter (100)/receiver(150) in a conventional OFDM mobile communication system.
The transmitter 100 includes an encoder 104, a symbol mapper 106, a serial-to-parallel (S/P) converter 108, a pilot symbol inserter 110, an IFFT module 112, a parallel-to-serial (P/S) converter 114, a guard interval inserter 116, a digital-to-analog (D/A) converter 118, and a radio frequency (RF) processor 120. In the transmitter 100, user data 102 including user data bits and control data bits to be transmitted is input to the encoder 104. The encoder 104 codes the user data using a corresponding coding scheme, and outputs the coded user data to the symbol mapper 106. The coding scheme includes a turbo coding scheme and a convolutional scheme having a predetermined coding rate. The symbol mapper 106 modulates coded bits output from the encoder 104 into modulation symbols using a corresponding modulation scheme, and outputs the modulation symbols to the S/P converter 108.
The modulation scheme includes BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16 QAM (16-ary Quadrature Amplitude Modulation), or 64 QAM (64-ary Quadrature Amplitude Modulation).
The S/P converter 108 parallel-converts serial modulation symbols output from the symbol mapper 106, and outputs the parallel-converted modulation symbols to the pilot symbol inserter 110. The pilot symbol inserter 110 inserts pilot symbols into the parallel-converted symbols, and outputs the pilot-inserted symbols to the IFFT module 112. The IFFT module 112 performs N-point IFFT on the signals output from the pilot symbol inserter 110, and outputs the IFFT-processed signals to the P/S converter 114.
The P/S converter 114 serial-converts the signals output from the IFFT module 112, and outputs the serial-converted signal to the guard interval inserter 116. The guard interval inserter 116 inserts a guard interval signal into the signal output from the parallel-to-serial converter 114, and outputs the guard interval-inserted signal to the D/A converter 118. The guard interval is inserted to remove interference between a previous OFDM symbol transmitted at a previous OFDM symbol time and a current OFDM symbol transmitted at the current OFDM symbol time in the OFDM communication system.
The guard interval has been proposed as a method for inserting null data for a predetermined interval. However, in the method for transmitting null data for the guard interval, when a receiver incorrectly estimates a start point of an OFDM symbol, interference occurs between subcarriers, thereby increasing false-alarm probability of a received OFDM symbol.
In order to solve this problem, one of the following two method is used: (1) a ‘cyclic prefix’ method for copying a predetermined number of last bits of an OFDM symbol in a time domain and inserting the copied bits into a valid OFDM symbol, and (2) a ‘cyclic postfix’ method for copying a predetermined number of first bits of an OFDM symbol in a time domain and inserting the copied bits into a valid OFDM symbol.
The D/A converter 118 analog-converts the signal output from the guard interval inserter 116, and outputs the analog-converted signal to the RF processor 120. The RF processor 120, including a filter and a front-end unit, RF-processes the signal output from the D/A converter 118, and transmits the RF-processed signal via a transmission antenna.
The receiver 150 has a reverse structure of the transmitter 100. The receiver 150 includes an RF processor 152, an analog-to-digital (A/D) converter 154, a guard interval remover 156, a serial-to-parallel (S/P) converter 158, an FFT module 160, a pilot symbol extractor 162, a channel estimator 164, an equalizer 166, a parallel-to-serial (P/S) converter 168, a symbol demapper 170, and a decoder 172. The signal transmitted from the transmitter 100 is received via a reception antenna of the receiver 150. The transmitted signal experiences a multipath channel and has a noise component.
The signal received via the reception antenna is input to the RF processor 152, which down-converts the signal into an intermediate frequency (IF) signal, and outputs the IF signal to the A/D converter 154. The A/D converter 154 digital-converts the signal output from the RF processor 152, and outputs the digital-converted signal to the guard interval remover 156.
The guard interval remover 156 removes a guard interval signal from the signal output from the A/D converter 154, and outputs the guard interval-removed signal to the S/P converter 158. The S/P converter 158 parallel-converts the serial signal output from the guard interval remover 156, and outputs the parallel-converted signals to the FFT module 160. The FFT module 160 performs N-point FFT on the signals output from the serial-to-parallel converter 158, and outputs the FFT-processed signals to the equalizer 166 and the pilot symbol extractor 162.
The equalizer 166 channel-equalizes the signals output from the FFT module 160, and outputs the channel-equalized signals to the P/S converter 168. The P/S converter 168 serial-converts the parallel signals output from the equalizer 166, and outputs the serial-converted signal to the symbol demapper 170.
Further, the signal output from the FFT module 160 is input to the pilot symbol extractor 162. The pilot symbol extractor 162 extracts pilot symbols from the signal output from the FFT module 160, and outputs the extracted pilot symbols to the channel estimator 164. The channel estimator 164 channel estimates the pilot symbols output from the pilot symbol extractor 162, and outputs the channel estimation result to the equalizer 166.
In addition, the receiver 150 generates channel quality information (CQI) corresponding to the channel estimation result of the channel estimator 164, and transmits the generated CQI to the transmitter through a channel quality information transmitter (not shown).
The symbol demapper 170 demodulates the signal output from the P/S converter 168 using a corresponding demodulation scheme, and outputs the demodulated signal to the decoder 172. The decoder 172 decodes the signal output from the symbol demapper 170 using a corresponding decoding scheme, and outputs the decoded signal as a final reception signal 174. The demodulation scheme and the decoding scheme correspond to the modulation scheme and the coding scheme used by the transmitter 100.
The OFDM-based system can also use Orthogonal Frequency Division Multiple Access (OFDMA) for multiuser access control. In OFDMA, users can use subsets of OFDM subcarriers using Frequency Hopping (FH) for spectrum spreading, and each subcarrier is exclusively allocated to one user each time. In this environment, radio resource allocation for performance optimization of the OFDMA system is very important.
In the OFDM mode, the most basic unit for resource allocation is an OFDM symbol, and the number of bits transmitted by one symbol is determined by the number of data carriers used per symbol, and also by modulation and coding schemes. In the OFDMA mode, the most basic resource allocation unit becomes a subchannel. Each OFDM symbol uses a multiple of subchannels according to a size of the FFT, and the number of data bits used per subchannel, which can be transmitted by each subchannel, is equal to the number of data carriers used per subchannel.
It is well known that in a wireless communication system having the OFDM-based physical structure, using channel state information secures effective application of an OFDMA system. However, the conventional channel allocation method is realized in a method of attempting channel allocation using a signal-to-noise ratio (SNR), which is measured for a predetermined time using state information of a radio channel.
Alternatively, as a simpler method, a method has been proposed for preferentially allocating the best channel using state information of a previous channel.
However, the conventional methods are disadvantageous in that an increase in the number of mobile stations causes an increase in overhead for exchanging channel information. In addition, if the number of mobile stations increases, the channel allocation method based on temporary SNR measurement shows the same performance as that of the existing random channel allocation method, thereby preventing efficient channel allocation.